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The tragedy happened a few days after an American researcher in criminal justice, Adam Lankford, released results of a global study of incidents involving 292 mass killers between 1966 and 2012. By way of definition, the murders in Virginia would not actually have made the survey as there were three victims - including the perpetrator himself - and the FBI determines a killing to be "mass" when it involves four or more deaths.

While the US is not the only country to experience the problem of mass shootings - Lankford's data came from 171 countries - it is certainly way out in the lead with 90 incidents. Only four other countries made it into double figures, as the graph below shows.

Britain has had three - Hungerford in 1987, Dunblane in 1996 and Cumbria in 2010. The US does not even have the most murderous mass shooting, that dubious distinction goes to Norway where Anders Breivik gunned down 69 young people at a summer camp outside Oslo in 2011.

An American phenomenon

Despite the global manifestation of mass shootings, they are frequently seen as a distinctly American phenomenon. The flipside of the American Dream, as it were, is the gun crazy nightmare and the instant "dark celebrity" that running amok with a firearm can achieve. And to understand that, argues Lankford, we have to understand the American gun culture, and the values that surround it.

America certainly leads the world in terms of its rate of gun ownership with almost 90 guns for every 100 people. The proportion of gun owners in the USA is falling, now around 35% - except among Republican leaning voters. Gun owners now tend to own more guns than they used to, choosing to augment their personal firepower as newer guns (military specification semi-automatic pistols and assault rifles) become available.

Globally, gun ownership is still a male preserve and the Geneva based Small Arms Survey project estimates that 96% of guns owners in the world are men. If anything, the US is slightly more equal, with 12% of women claiming to be gun owners. Revealingly, Lankford's survey finds only one mass killing perpetrated by a woman - the 2006 murder of six postal workers in California by Jennifer San Marco. Some research suggests that shooters specifically target women and girls as a form of violent abusive entitlement.

Clearly, as Lankford points out in his research, the risk of mass shootings is more likely in a society in which the private ownership of firearms is common as opposed to one where it is relatively rare. But rare events are incredibly difficult to predict. A particular kind of competitive individualistic culture may produce large numbers of embittered loners and losers bearing grudges and seeking revenge against former schools, employers, girlfriends, bullying classmates - all of which feature prominently in the diaries and social media epitaphs of perpetrators. But if these individuals can't access firearms, they can't undertake their outrages.

The real breakthrough in understanding came when better global data began to be produced of the relations between rates of gun ownership and gun violence from the late 1990s onwards. Before then, the US was the only real source of such data, although research into the impact of guns in the US has continued to be blocked by the gun lobby. More recent evidence shows a much clearer connection between rates of gun ownership and rates of gun homicide. Lankford's study comes to a similar conclusion:

Firearm ownership rates appeared to be a statistically significant predictor of the distribution of public mass shooters worldwide.

The power of gun culture

But the numbers are not the whole story: culture is important. There are relatively controlled "gun cultures" as well as decontrolled and dangerous ones, where you can legally purchase a lethal weapon from a total stranger in a car park, well below the radar of the authorities. Some 40% of US gun sales are thought to be "off the books" in this fashion.

It also matters what kinds of guns you have in your possession. Commentators have been noting that US rampage killings have lately become more frequent and more murderous, pointing to the contribution of assault rifles and large magazine capacity semi-automatics, which are now fairly widespread in civilian ownership. The timeline below shows mass shootings with more than 12 victims between 1945 and 2013.

But if mass shootings are not uniquely American in many respects, American rampage killing is distinctive in one important respect. In my own recent work, I've examined how mass shootings have often served as the catalyst for a society to finally grasp the nettle of gun control and bring sweeping changes to its gun laws: except in America.

A mermorial to the 28 victims of the Sandy Hook school shooting in Connecticut. Reuters/Eduardo Munoz

There is something even more uncomfortable about this too. A mass shooting, especially of children, can galvanise opinion into action much more than other types of violence. So 20, five-year-old white children, killed in a school in a single day can mobilise opinion more than the fact of many more African American kids being killed every month. The real "American exceptionalism" is not that it has had so many mass shootings, but rather that it has done so little about them.

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